As the field of wireless communications continues to develop at a rapid pace, the increasing number of systems and frequency bands in use conflicts with the consumer demand for increased mobility. That is, a problem results when different consumers subscribe to wireless communication systems using different technical standards. To avoid this problem, global standardized systems have been proposed in which the same frequency band is in use by all wireless communication systems. However, this solution is impractical because wireless communication systems are operators desire to maximize the return on their investment in the many different systems which already exist or which are already under development. Further, history suggests that it would be extremely difficult for all manufacturers to agree on a single standardized system.
An alternative solution to the problem of maximizing mobility where multiple wireless communication systems exist is to incorporate dual- or multi-mode capabilities in a wireless communication device to enable a subscriber to communicate on different wireless communication systems. This solution is particularly desirable where the different wireless communication systems operate at different carrier frequencies or frequency bands, but use the same modulation scheme and same baseband processing scheme. The well-known Global System for Mobile communications (GSM), Personal Communication Services (PCS), and Digital Communication System (DCS) systems share such similarities.
However, almost all known wireless communication devices are of the single-band type, because it is difficult to incorporate multi-band capabilities into a wireless communication device, given the size and cost limitations of typical devices. Further, most wireless communication devices use receivers which incorporate superheterodyne circuitry, in which received signals at a first frequency are converted to one or more second, intermediate frequencies for processing by the receiver. The intermediate frequencies can differ substantially from the first frequency. An exemplary known superheterodyne receiver is shown in FIG. 1, in which signals are received at an antenna 10, filtered in a band-pass filter 12, amplified in an amplifier 14, and converted to a first intermediate frequency by local oscillator LO1 and mixer 16. The signal at the first intermediate frequency is then processed by band pass filter 18 and amplifier 20, and converted to a second intermediate frequency by local oscillator LO2 and mixer 22. This second intermediate frequency signal is filtered in band pass filter 24 and is processed by further circuitry (not shown). Because a superheterodyne receiver processes signals at multiple frequencies, it is difficult to provide multi-mode capabilities without duplicating many receiver hardware components.
Accordingly, it would be desirable for a wireless communication device to be able to receive signals at multiple frequency bands while minimizing the duplication of receiver hardware.